The present invention relates generally to micro-electromechanical devices and, more particularly, to micro-electromechanical thermal actuators such as the type used in ink jet devices and other liquid drop emitters.
Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electrically resistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,209,989; 6,234,609; 6,239,821; 6,243,113 and 6,247,791. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,254,793; 6,258,284 and 6,274,056. The term xe2x80x9cthermal actuatorxe2x80x9d and thermo-mechanical actuator will be used interchangeably herein.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. Thermal actuators and thermal actuator style liquid drop emitters are needed which allow the movement of the actuator to be controlled to produce a predetermined displacement as a function of time. Highest repetition rates of actuation, and drop emission consistency, may be realized if the thermal actuation can be electronically controlled in concert with stored mechanical energy effects.
For liquid drop emitters, the drop generation event relies on creating a pressure impulse in the liquid at the nozzle, but also on the state of the liquid meniscus at the time of the pressure impulse. The characteristics of drop generation, especially drop volume, velocity and satellite formation may be affected by the specific time variation of the displacement of the thermal actuator. Improved print quality may be achieved by varying the drop volume to produce varying print density levels, by more precisely controlling target drop volumes, and by suppressing satellite formation. Printing productivity may be increased by reducing the time required for the thermal actuator to return to a nominal starting displacement condition so that a next drop emission event may be initiated.
Apparatus and methods of operation for thermal actuators and DOD emitters are needed which enable improved control of the time varying displacement of the thermal actuator so as to maximize the productivity of such devices and to create liquid pressure profiles for favorable liquid drop emission characteristics.
A useful design for thermo-mechanical actuators is a cantilevered beam anchored at one end to the device structure with a free end that deflects perpendicular to the beam. The deflection is caused by setting up thermal expansion gradients in the beam in the perpendicular direction. Such expansion gradients may be caused by temperature gradients or by actual materials changes, layers, thru the beam. It is advantageous for pulsed thermal actuators to be able to establish the thermal expansion gradient quickly, and to dissipate it quickly as well. It is further beneficial to actively generate opposing thermal expansion gradients to assist in restoring the actuator to its initial position. This may be achieved by having dual actuation means operating to deflect a cantilevered beam in substantially opposite directions.
A dual actuation thermal actuator configured to generate opposing thermal expansion gradients, hence opposing beam deflections, is useful in a liquid drop emitter to generate pressure impulses at the nozzle which are both positive and negative. Control over the generation and timing of both positive and negative pressure impulses allows fluid and nozzle meniscus effects to be used to favorably alter drop emission characteristics.
Cantilevered element thermal actuators, which can be deflected in controlled displacement versus time profiles, are needed in order to build systems that can be fabricated using MEMS fabrication methods and also enable liquid drop emission at high repetition frequency with excellent drop formation characteristics.
It is therefore an object of the present invention to provide a thermal actuator which comprises dual actuation means that move the thermal actuator in substantially opposite directions allowing rapid restoration of the actuator to a nominal position and more rapid repetitions.
It is also an object of the present invention to provide a liquid drop emitter which is actuated by a dual activation thermal actuator configured using a cantilevered element.
It is further an object of the present invention to provide a method of operating a thermal actuator utilizing dual actuations to achieve a predetermined resultant time varying displacement.
It is further an object of the present invention to provide a method of operating a liquid drop emitter having a thermal actuator utilizing dual actuations to adjust a characteristic of the liquid drop emission.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by constructing a thermal actuator for a micro-electromechanical device comprising a base element and a cantilevered element extending from the base element and normally residing at a first position before activation. The cantilevered element includes a barrier layer constructed of a low thermal conductivity material, bonded between a first deflector layer constructed of a first electrically resistive material having a large coefficient of thermal expansion and a second deflector layer constructed of a second electrically resistive material having a large coefficient of thermal expansion. The thermal actuator further comprises a first pair of electrodes connected to the first deflector layer to apply an electrical pulse to cause resistive heating of the first deflector layer, resulting in a thermal expansion of the first deflector layer relative to the second deflector layer. A second pair of electrodes is connected to the second deflector layer to apply an electrical pulse to cause resistive heating of the second deflector layer, resulting in a thermal expansion of the second deflector layer relative to the first deflector layer. Application of an electrical pulse to either the first pair or the second pair of electrodes causes deflection of the cantilevered element away from the first position to a second position, followed by restoration of the cantilevered element to the first position as heat diffuses through the barrier layer and the cantilevered element reaches a uniform temperature.
The present invention is particularly useful as a thermal actuator for liquid drop emitters used as printheads for DOD ink jet printing. In this preferred embodiment the thermal actuator resides in a liquid-filled chamber that includes a nozzle for ejecting liquid. The thermal actuator includes a cantilevered element extending from a wall of the chamber and a free end residing in a first position proximate to the nozzle. Application of an electrical pulse to either the first pair or the second pair of electrodes causes deflection of the cantilevered element away from its first position and, alternately, causes a positive or negative pressure in the liquid at the nozzle. Application of electrical pulses to the first and second pairs of electrodes, and the timing thereof, are used to adjust the characteristics of liquid drop emission.